In a cellular wireless communication system, geographical conditions in a cell, a distance between a User Equipment (UE) and a base station, or movement of the UE may deteriorate a channel condition and block the communication between the UE and the base station. For example, even in the service coverage of the base station, a sealed structure such as office or house can form a shadow area. When the UE travels in such a shadow area, the base station cannot normally communicate with the UE because of the poor channel condition in relation to the UE.
In this respect, the wireless communication system can provide a femtocell service to address the service problem in the shadow area and to offer a high-rate data service. Hence, the femtocell indicates the service coverage of a compact base station which accesses a mobile communication core network via an indoor broadband network. The compact base station, which is installed in office or indoors to limit the access of the unauthorized user, can be called a self-configurable base station, a home base station, a femto base station, and a Closed Subscriber Group (CSG) base station. Hereinafter, the compact base station is referred to as the femto base station.
The femto base station works with a macro base station, and the UE may need to be serviced as it hands over between the femto base station and the macro base station. That is, the UE may need to be provided with improved service as it hands over between the macro base station and the femto base station according to the radio condition.
One macrocell can include a plurality of the femtocells. A Physical Cell ID (PCI), which is one type of ID for identifying the cell, can be duplicated for the macro cell and the femtocells, except for a specific PCI allocated only for the femtocell. Hence, when detecting the same PCI as the allowed femtocell through a cell search procedure during the movement, the UE cannot determine whether the detected PCI is the PCI allocated to the allowed femtocell and accordingly can hardly measure the cell corresponding to the detected PCI. This is a matter of great concern when only the UE allowed to access a particular femtocell is handed over to the corresponding femtocell. Thus, a Long Term Evolution (LTE) standard defines a Cell Global ID (CGI) which is the unique cell ID for identifying the femtocell, in addition to the PCI, and supports the mobility of the UE using the CGI.
Herein, a conventional method for the UE to report the PCI and the CGI to the macro base station and for the macro base station to support the mobility of the UE using the PCI and the CGI is discussed now.
The UE receives a Primary Synchronization Signal (PSS) and a Secondary Synchronization Signal (SSS) from its neighbor cell and obtains the PCI allocated to the neighbor cell. Since it is hard to identify the femtocell merely with the PCI, the UE receives a Master Information Block (MIB) and a System Information Block (SIB) from the corresponding neighbor cell and obtains the CGI which is the unique cell ID of the corresponding neighbor cell. Next, the UE generates a measurement report message including the PCI and the CGI obtained from the neighbor cells and sends the generated message to its serving macro base station. The serving macro base station determines based on the measurement report message whether the corresponding neighbor cell is the cell accessible by the UE, and instructs the UE to hand over to the corresponding neighbor cell according to the determination result.
As such, as the UE has to obtain the CGI of the neighbor cell because of the overlapping PCI, the measurement time for obtaining the PCI and the CGI is lengthened and thus the performance of the UE is deteriorated. In particular, when a number of femtocells are present around the UE, the UE may need to repeatedly receive the MIB and the SIB of each femtocell, which may degrade the performance of the UE even more.